Uses of dipole magnet devices capable of achieving very high magnetic dipole fields may include the bending of particle beams in synchrotrons, NMR imaging for scientific and medical analysis, wind power generation and the bending of particle beams in devices used for ion therapy.
Synchrotrons are particle acceleration devices that have been conventionally used to study high energy physics phenomena. Beams of particles are accelerated and bent into two nearly circular paths and are then brought into collision. Head-on collisions of individual particles may create new particles of scientific interest via the equation E=mc2. Contemporary devices are large, with circumferences of tens of kilometers. In order to achieve significantly higher beam energies, one must increase either the circumference of the synchrotron or the magnetic field of its dipole magnets.
In the first synchrotrons, normal-conducting wires were used in conjunction with magnetic materials to form the dipole magnets, and fields were typically in the vicinity of 2 teslas. A significant advance in making larger dipole magnetic fields is described by Blosser and Milton (U.S. Pat. No. 4,641,057, February 1987), which teaches the use of superconducting dipole magnet technology for synchrotrons. In order to achieve even higher fields, high-temperature-superconducting materials have proven valuable. Aized and Schwall (U.S. Pat. No. 5,525,583, June 1996 and U.S. Pat. No. 5,914,647, June 1999) teach how control of the geometry of high-temperature-superconducting-tape can lead to an increase in the carrying capacity and center magnetic field produced by a high-temperature-superconducting-coil. Kodenkandath, et al. (U.S. Pat. No. 7,781,376, August 2010) teach how use of two layers of high-temperature-superconducting material, each of which is selected for its performance at a particular magnetic field direction and which, together, result in enhanced performance for high-temperature-superconducting-coils. Superconducting coils have enabled higher magnetic fields than what was achievable using earlier magnet technology. Antaya et al. (U.S. Pat. No. 7,656,258, February 2010) teach how to obtain magnetic fields of at least 9.9 teslas using low-temperature-superconducting-coils, and Antaya et al. (U.S. Pat. No. 8,614,612 December 2013) teach how to obtain magnetic fields in excess of 14 teslas using high-temperature-superconducting-coils. Hence, conventional techniques in dipole magnet devices include the use of normal conductors, low temperature superconductors and high temperature superconductors. Conventional techniques also include liquid helium systems to cool the coils, support structures to support the coils against Lorentz forces present in the system, electrical contacts to allow electric current into and out of the coils, and an open region for particle beam transport and other uses. These conventional techniques are presently limited to fields less than or equal to about 15 teslas, and control of erroneous fields can be difficult.
One of the issues in moving beyond 15 teslas involves persistent-current magnetization. Persistent-current magnetization may produce an unwanted distortion of the magnetic field in superconducting tape. This magnetization is proportional to the width of the superconductor that is perpendicular to the magnetic field. In the case of a high temperature superconducting (HTS) tape, the HTS tape may have one wide dimension for its tape face such as for example, 2 mm to 12 mm, whereas the thickness may be less by approximately three orders of magnitude: 0.5 microns to 5 microns (0.0005 mm to 0.005 mm). Therefore, with these dimensions, the HTS tape has a large asymmetry. The dimensions of the HTS tape are different from superconductors that are made with round wires (such as niobium titanium, niobium tin and Bi2212) where such a large asymmetry does not exist.
Examples of superconductor tape materials include low temperature superconductors (LTS) and high temperature superconductors (HTS). LTS materials such as NbTi and Nb3Sn may be cooled to about 4 K to become superconducting. HTS materials may become superconducting above 77 K. HTS in the form of a tape geometry (such as Bi2223 and ReBCO) in magnets may be subject to distortion in field uniformity due to the large magnetization (due to persistent-currents).
Early commercially available HTS materials were bismuth-based ceramic oxides featuring Bi-2223 and are sometimes referred to as first-generation HTS. Second-generation HTS materials have been developed using rare earth barium copper oxide ceramics. The rare earth element may be one or more of yttrium, samarium, or gadolinium. These HTS materials are commercially available in the form of a thin flat tape and are also referred to as multi-layer coated conductors. HTS tape may be used in many applications and devices, for example, superconducting magnetic energy storage (SMES) devices, particle accelerators and medical applications.
The current carrying capacity of the HTS tape is highly anisotropic. The current density when the field is parallel with the tape wide face is several times the value when the field is perpendicular to the tape wide face. As a result of this observation, J. van Nugteren, et al. (“Study of a 5 T Research Dipole Insert-Magnet using an Anisotropic ReBCO Roebel Cable,” IEEE Transactions on Applied Superconductivity, 15 Oct. 2014) teach that aligning the tape may reduce the amount of expensive tape. Another characteristic of the tape conductor geometry is that large magnetization currents are generated when the magnetic field is perpendicular to the wide face of the tape. Since the magnitude of the magnetization current is proportional to the dimension of the conductor that is perpendicular to the applied magnetic field, the magnetization current is highly anisotropic for a tape type conductor where the wide face dimension is 2-12 mm and the thickness is 0.02-0.04 mm. Conventional cosine theta magnet designs and common coil design may be expected to generate large field errors, because the wide dimension of the tape remains mostly perpendicular to the field and this orientation generates large persistent currents. The geometry of the conductor is a major factor to the development of these field errors. One method that may reduce the distortions is the use of a Roebel cable available commercially. With this approach, the tape is cut into a pattern that allows several tapes to be nested together. This reduces the effective magnetization from the original width of the tape, e.g. 12 mm, to the width of the pattern, e.g., 2 mm. However, this is achieved by cutting and discarding about 50% of the original superconductor tape, which is very expensive. Also, the reduction in distortion achieved is limited to the ratio of tape width to pattern width.
There is an interest in superconducting magnets made with tape geometry because of significant advances in High Temperature Superconductors (HTS). Bi2212 is primarily available in round wire form, but Bi2223 and ReBCO are commercially available in tape form. Because of the high strength and large production of ReBCO (and Bi2223), tape conductors have generated interest in accelerator and other applications.
Achieving dipole magnet fields in the range of 16-25 or 19-25 teslas at high quality has proven difficult using conventional techniques.
Accordingly, there is a need for an improved method and system for generating magnetic dipole fields that will overcome the field limit of conventional techniques by providing magnetic fields in the range of 16-25 or 19-25 teslas at high quality while overcoming the problem presented by persistent-current magnetization effects.